Cosmological perturbations from vector inflation

TL;DR

This work analyzes linear cosmological perturbations in vector-driven inflation, where multiple vector fields with non-minimal gravity couplings drive a quasi-de Sitter expansion. The authors derive the full set of coupled scalar, vector, and tensor perturbation equations, demonstrate UV decoupling for canonical quantization, and examine superhorizon evolution in the small-field, large-N regime. They find that vector perturbations of the metric decay exponentially while scalar and tensor modes remain weakly coupled, enabling potentially observable correlations between scalar and tensor modes in the CMB, though the effect is small in realistic models. The results establish vector inflation as a viable alternative to scalar inflation with distinctive observational signatures, motivating future tests for anisotropy and scalar–tensor correlations in the CMB.

Abstract

We analyze the behavior of linear perturbations in vector inflation. In contrast to the scalar field inflation, the linearized theory with vector fields contains couplings between scalar, vector and tensor modes. The perturbations decouple only in the ultraviolet limit, which allows us to carry out the canonical quantization. Superhorizon perturbations can be approximately analyzed due to suppressed mixing between different modes in the small fields models. We find that the vector perturbations of the metric decay exponentially, but the scalar and tensor modes could remain weakly coupled throughout the evolution. As a result, the vector inflation can produce significant correlations of the scalar and tensor modes in the CMB. For the realistic models the effect is rather small, but not negligible.